The present invention relates to a method of manufacture of a shaped object from a composite material such as a composite material reinforced with reinforcing material, and more particularly relates to a method of manufacture of such a shaped object from a composite material which employs plastic processing, and in which the composite material is brought to be partially or totally in the molten state before the plastic processing, in order to render the ease of the plastic processing greater.
In motor vehicles and aircraft and so forth, nowadays, the constant demand for lightening and strengthening and providing higher performance of structural members and parts, with the objects, among others, of providing energy savings by reducing fuel consumption, of increasing durability, and also of providing higher traveling speeds, has meant that construction from light metallic alloys such as aluminum alloy or magnesium alloy has become common. Problems arise, however, in making parts from light metallic alloys, despite the light weight of these alloys, and despite their easy workability, because the mechanical characteristics of these alloys such as wear resistance, and such as strength including bending resistance, torsion resistance, tensile strength, and so on, are inferior to those of competing materials such as steel. Further, the occurrence of cracking and the spreading of cracks in parts made of aluminum or magnesium or other light metal alloys can be troublesome. Therefore, for parts the strength and/or wear resistance of which is critical there are limits to the application of light metallic alloys.
Accordingly, for such critical members, it has become known and practiced for them to be formed out of so called two phase or composite materials, in which reinforcing material is dispersed within a matrix of metal. If the matrix metal is an aluminum or magnesium alloy, for example, then the advantages with regard to weight of using this aluminum or magnesium alloy as a constructional material can be obtained to a large degree, while avoiding many of the disadvantages with regard to low strength and crackability; in fact, the structural strength of the composite materials made in this way can be very good, and the presence of the reinforcing material can stop the propagation of cracks through the aluminum or magnesium alloy matrix metal.
The reinforcing material conventionally has been known as for example being alumina fibers, carbon fibers, silicon carbide whiskers, or possibly mixtures thereof, and the matrix metal has been known as for example being various types of aluminum or magnesium alloy, as mentioned above; and various proposals have been made with regard to compositions for such fiber reinforced metal type composite materials, and with regard to methods of manufacture thereof.
With regard to methods of manufacture of composite materials, various methods of manufacture have been tried, such as the high pressure casting method, the centrifugal casting method, the die cast method, the low pressure casting method, and the autoclave method. But of these the most generally and usefully applicable has so far been the high pressure casting method, in view of the low cost of the fiber reinforced metal type composite material produced thereby, and the manufacturing efficiency attained thereby. In this per se well known high pressure casting method, a mass of reinforcing fibers is placed in the mold cavity of a casting mold, and then a quantity of molten matrix metal is poured into the mold cavity. The free surface of the molten matrix metal is then pressurized to a high pressure such as approximately 1000 kg/cm.sup.2 by a plunger or the like, which may be slidingly fitted into the mold. Thereby the molten matrix metal is intimately infiltrated into the interstices of the mass of reinforcing fibers, under the influence of this pressure. This pressurized state is maintained until the matrix metal has completely solidified. Then finally, after the matrix metal has solidified and cooled into a block, this block is removed from the casting mold, and the surplus matrix metal around the reinforcing fibers may be removed by machining, so that the composite material mass itself, consisting of the mass of reinforcing fibers impregnated with matrix metal, is isolated. This high pressure casting method has the advantage of low cost, and it is possible thereby to manufacture pieces of composite material of different shapes including quite complicated shapes with high efficiency.
Now, however, since such reinforcing fiber materials are vastly harder than the aluminum or magnesium alloy matrix metal in which they are embedded, therefore finishing operations for parts or other objects formed of such composite materials including such reinforcing fiber materials become much more difficult. Further, during use of the parts, other problems occur: for example, the wear on mating or cooperating members which rub against such parts, sliding relatively to them, may become very great.
Specifically, the principal methods for forming a finished part or other object of such a composite material fall into the two general categories of machining processes and plastic processes.
In the case of machining processes, when a part preform or roughly manufactured object made from such composite material is subjected to finishing machining, since the reinforcing material is very much harder than the matrix metal, and in the case of for example silicon carbide whiskers has a hardness second only to diamond, this machining is very difficult and expensive, and great wear on the machining tools is caused; and also the resulting machined surfaces on the finished object are often very rough and poorly finished, which cauess the problems associated with wear on a mating or cooperating member to be even more particularly marked. Further, the existence of such rough finished surfaces means that it can often occur during rubbing of the finished composite material part against a mating member that reinforcing particles of the finished composite material part can become dislodged from the matrix metal in which they are embedded, especially large sized such reinforcing particles, and thus scuffing of the material of such a mating or cooperating member may well occur, which can cause great damage to such a member. Thus machining finishing of a composite material object or part is expensive and not suited for volume production.
On the other hand, in the case of plastic processes for forming objects from composite material, such as forging or extrusion processes, when these are applied to the composite material in substantially the cold state, since the plastic flowing of the composite material under stress is impeded by the presence of the composite reinforcing material therein, the process cannot be performed at an acceptable rate, and other problems arise, such as that very high pressure is required, or that a particular desired orientation which has been set up for the reinforcing fibers or other reinforcing material in advance is disturbed, or that some of the reinforcing fibers are broken.
Because of the above described type of difficulties which occur when applying plastic processing for forming objects from composite material, it has been considered to apply the process of applying to the composite material a semi molten type plastic process, of a type which have been developed for materials which are difficult to process. In such a semi molten type of process, the composite material is heated up so that the matrix metal thereof is partly in the molten state, and thus it is easier to deform plastically. But, as the liquid or molten phase proportion of the matrix metal is increased, handling of the composite material becomes more and more difficult; and, as described for example in U.S. Pat. No. 3,668,748, it is impracticable to increase the liquid phase proportion of the matrix metal to above 25%. Thus, with such a semi molten type of processing method for forming objects out of composite material, in the prior art it has not been possible to perform production as satisfactorily, or as quickly, as could be wished; and further the problems described above with regard to the cold type of plastic process are not fully resolved.